Once a server is promoted to a DC (i.e the DC role is installed), the "default" administrator account you mention no longer exists. The Administrator account now exists in the domain. Any account created on the DC from now on will exist only on the domain. The DC no longer has the concept of "Local Users".
The fundamental reason that a DC should have other standard user accounts set up is because you don't want your users to be using the administrator account, and you want each user to have their own identity.
Consider creating your own standard user account, and only use the administrator account when absolutely needed.
Also worth a mention, Best practice is to rename the administrator account